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New Scientific Applications with Existing CGPS Capabilities
Earthquakes, Soil Moisture, and Environmental Imaging
Andria BilichGeosciences Research DivisionNational Geodetic Survey
Overview
New uses of existing geodetic networks and stations (CORS, IGS, etc.)
Earthquakes / seismograms High-rate GPS Example: 2002 Denali Fault event
Soil moisture Near-field multipath Example: Uzbekistan
Environmental imaging Near- and far-field multipath Examples: Mauna Kea and Canada
Earthquakes withHigh-Rate GPS GPS/GNSS positioning
No upper limit to amplitude No preset ‘frequency response’ Positions can be computed at every data
epoch, independently Precise and accurate displacements Well-defined reference frame
Earthquakes Static and transient deformations Potentially large magnitude Frequencies = seconds to hours
GPS Data Rates and Analysis Strategies
Traditional Traditional Geodetic GPSGeodetic GPS
High-rate GPSHigh-rate GPS
Long period
(days to years)Signal
Short period
(seconds to days)
30 seconds Sample rate 1 Hz or higher
5 minutes Decimation None
1 per dayPosition
estimatesEvery sample
28+ satellitesSatellites in
solution6-8 satellites
Denali Earthquake2002 November 3
USGS fact sheet
USGS fact sheet
Long strike-slip rupture
Magnitude 7.9
Shallow SE directivity Large
surface waves
Clipped Seismometers+ 1-Hz GPS
Many broadbands in western North America went off scale…
… and high-rateGPS fills in the gaps
Take-home lessons:High-rate GPS/GNSS GPS and seismometers have
complementary strengths/weaknessesNoisy GPSOff-scale seismometers
Possible only through GNSS technology advances: data storage, chipsets, firmware, etc.
Existing HR GPS networks expanding…
Multipath Background
What is multipath? Site-specific Time-varying Sensitive to
environmental changes How can we measure
multipath? Pseudorange data
combination Solution residuals Signal-to-noise ratio
Signal-to-Noise Ratio (SNR)
Measure of signal strength Total SNR = direct plus reflected signal(s)
Direct amplitude = dominant trend Multipath signal = superimposed on direct
Soil Moisture from Near-Field Multipath
Existing GPS stations! Ground reflections
Amplitude attenuation at ground
Soil moisture affects attenuation (reflection coefficient)
Method = monitor SNR amplitude changes over time
Larson et al., GPS Solutions, 2007.
Take-home lessons:Soil Moisture Possible to use existing CGPS monuments
and receivers SNR always computed, sometimes reported S1,S2 = archived in RINEX
Challenges and issues: SNR data quality Antenna gain pattern effects Satellite power Vegetation, temperature effects Sensing depth and footprint
Environmental Imaging with Near- & Far-field Multipath Extension of soil moisture principles…
SNR data Reflection strength from multipath amplitude
… plus frequency content of SNR Satellite motion creates time-varying
signature h (fast = far; slow = close)
Power spectral maps Frequency and amplitude with respect to
satellite position (elevation/azimuth) Projected onto map of antenna environment
MKEA Power Maps
Long periods at low satellite elevation angles
Shorter periods at high elevation angles
High power returns from cinder cones
60-90s 30-60s 10-30s
Dual-FrequencyPower Spectral Maps
S1 S2
Reflection from distant object (building?)
Reflection from nearby object (rock outcrops?)
Churchill (CHUR),
Manitoba, Canada
Take-home lessons:Environmental Imaging Assess multipath environment
Frequency: distance to object Amplitude: magnitude of errors due to object Consider position errors at different
frequencies (think high-rate GPS positioning) No new equipment
SNR routinely recorded … but need precise and accurate SNR
related to multipath model (not always possible)